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Abstract

Self-assembled, sub-wavelength periodic structures are induced in fused silica by a tightly focused, linearly polarized, femtosecond laser beam. Two different types of periodic structures, the main one with period (ΛE) in the direction of the laser beam polarization and the second with period (Λk) in the direction of the light propagation, are identified from the cross-sectional images of the modified regions using scanning electron microscopy. We demonstrate the spatial coherence of these nanogratings in the plane perpendicular to the beam propagation direction. The range of effective pulse energy which could produce nanogratings narrows when the pulse repetition rate of writing laser increases. The period ΛE is proportional to the wavelength of the writing laser and period Λk in the head of the modified region remains approximately the wavelength of light in fused silica.

Figures (7)

Fig. 1. Modified regions in sample A. E: electric field of the writing laser, k: wave vector of the writing laser beam, line Ax and line Ay: Ep=0.48 µJ, V=200 µm/s, λ=850 nm. (a): Schematic of the sample A showing how the self-assembled periodic structures are expected to be arranged. n1 and n2: local refractive indices of the plates of thickness t1 and t2, respectively. (b): SEM images of Ax and Ay in the xz plane. (c): Detail of the sub-wavelength periodic structure formed in the cropped region of Ax.

Fig. 4. Microscope images of the irradiated regions in the xy plane in sample C, image on the left taken by back-illumination without polarizers, image on the right taken by back-illumination with cross polarizers of the same irradiated area. (a): Rep=500 kHz, V=500 µm/s. (b): Rep=1 MHz, V=1000 µm/s.

Fig. 5. (a): SEM image of a written line in the xz plane, E: electric field of the writing laser, k: wave vector of the writing laser beam (λ=800 nm, τp=50 fs, Ep=0.5 µJ, V=100 µm/s, Rep=250 kHz). (b): The region between two dotted lines is used for calculation. Corresponding normalized correlation functions calculated along the x (c) and z (d) axis.